The invention relates generally to an apparatus for determining fluid level and flow velocity in a single-phase and/or two-phase fluid system, and in particular to an apparatus for determining fluid level and flow velocity in a downcomer of a natural recirculation boiling water reactor (BWR) using a combination of electrical conductivity (EC) probes, thermal conductivity (TC) probes and one or more time-domain reflectometry (TDR) probes.
Boiling water nuclear reactors generally comprise steam-generating plants in which reactor water coolant is circulated through a core of heat-producing fissionable nuclear fuel to transfer thermal energy from the fuel to the coolant, thereby generating a two-phase steam-water mixture emerging from the fuel core. Using steam-water separators and steam dryers positioned downstream from and above the core, the upward-flowing mixture from the heating core becomes partitioned into its respective phases, whereupon the steam is piped from the reactor vessel for use in steam-driven turbines or other equipment while the liquid water phase is recycled as coolant water.
In typical boiling water reactors used for power generation, reactor coolant water is circulated continuously around a flow path as follows: up through a heat-producing fuel core; then up through an upper outlet plenum superimposed above the fuel core which serves to collect and channel all the coolant passing up through the fuel core. Then, the coolant water passes through an assembly of steam separators positioned above the core outlet plenum; and then travels finally back downward outside of the core, along an annular region, known as the “downcomer,” to recycle the liquid coolant and return it to the fuel core.
It has previously been known that the level of a liquid can be determined using electrical conductivity (EC) probes. In such a conductivity probe, several waveforms can be used for interrogation. In one example, a constant voltage (AC) is imposed across a gap between two electrodes. The magnitude of the resulting current is determined by the ability of the medium to conduct the current, where admittance is the reciprocal of impedance. In another example, a constant voltage (DC) is imposed across a gap between two electrodes. The magnitude of the resulting current is determined by the ability of the medium to conduct the current, where conductance is the reciprocal of resistance.
In a TDR-based level measurement device, one or a series of low-energy electromagnetic impulses generated by the sensor's circuitry is propagated along a thin wave guide (also referred to as a probe)—usually consists of one single long electromagnetic wave conductor or an array of long conductors, such as a metal rod, a steel cable, or a metal thin tube with a coaxially fixed metal rode in the middle. When these impulses hits the surface of the medium to be measured, an impedance mismatch (due to the different dielectric constants of the two phases) causes part of the impulse energy to be reflected back up the probe to the circuitry (due to the mismatch of the dielectric property) which then calculates the fluid level from the time duration between the impulse sent and the impulse reflected (in nanoseconds).
In the operation of such natural circulation nuclear reactors, the maximum power per fuel assembly unit critically depends upon this recirculation coolant flow through the fuel core. In addition, significant bundle natural circulation flow, which is nearly compatible to that of forced circulation BWR designs, is achieved in an Economic Simplified Boiling Water Reactor (ESBWR) design. Thus, it is desirable to accurately measure the flow rate of this recirculation flow through the downcomer of the nuclear reactor, particularly in a natural circulation BWR.
It would therefore be desirable to provide a system and method for measuring in-core fluid level and flow velocity in a multiple phase fluid system, such as in a boiling water nuclear reactor.